In this research project, the design and development of TiO_2 photocatalysts which can absorb visible light and operate efficiently and effectively not only under ultraviolet light but also upon irradiation with visible light or solar beam for the decomposition of NO into N_2 and 0_2 were carried out. Until now, photocatalytic reactions involving TiO_2 photocatalysts for the decomposition reaction of nitrogen oxides (NOx) in the atmosphere are known to proceed only under irradiation of TiO2 photocatalysts with ultraviolet light having shorter wavelength than 380nm, allowing the use of 3% of solar beam. In order to use solar beam more efficiently, we have elucidated the modification of TiO_2 photocatalyst by applying the advanced metal ion-implantation with various transition metal ions such as Cr highly accelerated by high voltage. A combination of such metal ion-implantation and the subsequent calcination of the catalyst in oxygen at around 723 K enables a large shift in the absorption bands of TiO_2 toward visible-light regions, its extent depending on the amount of Cr ions implanted. Such Cr ion-implanted TiO_2 catalysts successfully decompose NO into N_2, 0_2 and N_2O even at 275 K under irradiation with visible light longer than 450nm. Furthermore, the reaction proceeded at exactly the same rate to that of the unimplanted TiO_2 catalysts under band-gap UV light irradiation. Under outdoor solar light irradiation at ordinary temperatures, the Cr ion-implanted TiO_2 catalyst exhibited photocatalytic reactivity several times higher than the original unimplanted TiO_2 photocatalyst. It can be said that present research has opened the way to many innovative possibilities, significantly to address urgent environmental problems, and can also be considered an important breakthrough in the utilization of solar energy.